Abstract: The calculation and analysis on three special breathing rates of air flow of oil tank have been made. The results have shown that for the air temperature under abrubt reduction condition, the value difference between the breathing under the stand pressure and the rate of air flow supplied is too big, which leads to pressure dropping and results in collapse of oil tank. This paper introduces a new two-step breathing valve which can meet all requirements of the tank's breathing. When internal pressure of the tank changes abruptly, the valve may completely open quickly to satisfy all requirements of the tank's breathing. So it can ensure the safety of the tank's breathing.